Aspects and embodiments of the invention most generally pertain to distance measuring sensors; more particularly to a chip-scale, distance measuring photonic integrated circuit; and, most particularly to a distance measuring pixel and associated coherent detection method for determining a measured distance from the distance measuring pixel to a remote object.
Miniaturized imaging systems capable of high precision 3D range measurements are of great interest as sensors in applications such as industrial inspection, construction and architecture, Virtual Reality, and autonomous vehicles. Most systems today are based on the measurement of the time of flight (ToF) of light, phase shift of a time varying light signal, or optical triangulation: either by measuring the distortion of a projected laser pattern on a surface to determine its 3D shape, or using two cameras to simulate stereo vision.
ToF systems measure the delay between an emitted pulse of light and its reflection from a distant object point to infer its distance. Since the speed of light is a known constant the calculation of distance is trivial based on the time between emitted and received pulses. However, the major drawbacks of such a system are the need for very high speed precision measurement circuits and large laser emission power required.
In the phase-shift method, the transmitted light intensity is modulated sinusoidally, and the phase shift of the reflected light signal from round-trip time is used to determine distance. Drawbacks of the phase shift method include range dependence on modulation frequency, susceptibility to crosstalk and noise, and a need to collect many samples to average the error between measurements and recover an accurate distance.
Both ToF and phase-shift systems need to sweep the spot from one or multiple lasers over a surface to adequately estimate its 3D shape. Current laser scanning range finding systems are expensive due to the need for precision optomechanical assembly, and high resolution motors capable of accurately rotating the laser measurement system and 3D mapping a surface or environment. These systems however, provide the highest resolution of distance measurement and 3D mapping capability over long distance ranges, indoors and outdoors.
Sensors that employ light patterns are usually smaller and inexpensive. Since they rely on the analysis of distortions in the emitted light patterns to map the shape of a surface they are generally lower in power, do not require the use of high speed precision measurement circuits, and may not need to be mechanically swept over the surface of interest. The major limitations of these sensors are poor range, resolution, and inability to operate in outdoor and bright ambient environments. Both types of devices also suffer from generally slow response.
A more recent invention is a Photonic Mixing Device (PMD) (U.S. Pat. No. 7,361,883), which is a variant of a radio frequency modulated (RFCW) continuous wave LIDAR. In this sensor the complex electrical mixing circuitry commonly used in RFCW LIDAR is replaced with all optical mixing that occurs within the pixel of a photosensitive semiconductor device. A modulated light source is used to illuminate a surface. The photosensitive semiconductor surface is electrically modulated at the same frequency as the illumination source and 3D distance is determined by the electro-optical mixing of the two signals.
Although miniature and inexpensive, the drawback of the PMD device is the need for large pixel sizes to capture adequate reflected light resulting in poor lateral resolution of a depth image. The limited frequency at which the photosensitive semiconductor pixels in the system can be modulated during optical mixing also limits the device's depth measurement resolution. Furthermore, the system is susceptible to saturation from ambient light, which introduces errors in distance measurement.
Coherent methods for laser ranging, such as frequency modulated continuous wave (FMCW) ranging, which uses optical homodyne detection can provide improvements in range, distance measurement accuracy, and operation in ambient lighting conditions over all of the other light ranging methods described. In such a system, the frequency of emission of the light is linearly modulated and the beam is split into two, wherein one beam is directed towards a measurement target, while the other forms a local oscillator (LO). Light returning from the target is mixed with the LO beam on the surface of a photodetector to provide optical interference patterns which may be processed to provide detailed range information about the target. On a square-law point detector such as a p-i-n photodiode, these interference fringes are manifest as a unique beat frequency, at which the detector's photocurrent is modulated, that is proportional to the modulation frequency of the light as well as the distance to the target. Such systems are described in U.S. Pat. No. 7,139,446B2 and U.S. Pat. No. 8,687,173B2.
FMCW systems are capable of providing precise distance measurements using low laser power levels. These systems may be coupled to rotating mirrors to scan the light over surfaces and produce range maps of environments. However, such systems are usually relatively expensive due to the complexity of the optical architecture and precise optical tolerances required in embodiments using free-space optics. Embodiments of such systems may also be implemented using fiber optic components, however these tend to be bulky and only slightly less complicated than free-space designs to manufacture.
What is needed therefore, is a distance measuring sensor that does not trade off measurement resolution and cost, is not susceptible to environmental artifacts, retains a compact size, and can be manufactured inexpensively. The following disclosure describes an FMCW distance measurement sensor that meets these requirements.